Solar Energy Collection Apparatus and Method

ABSTRACT

An apparatus for collecting heat from a solar concentrator has an isothermal body defining an elongated cavity with a circular opening having a diameter equal to a diameter of a focus of the solar concentrator, the cavity having a reflective walls such that solar rays contacting the walls are substantially reflected. The circular opening is located at the focus of the solar concentrator and perpendicular to a principal axis of the solar concentrator, and the axis of the cavity is aligned with the principal axis of the solar concentrator. The heat generated in the isothermal body is absorbed by the heat sink. The length of the cavity is sufficient to absorb a desired proportion of the energy in the solar rays entering the cavity and is about 5 to 9 times the diameter of the opening of the cavity. Depending on material used, the isothermal body can be enclosed in a reducing atmosphere to maintain reflectivity of the cavity walls.

This invention is in the solar energy field and in particular thecollection of concentrated solar radiation for the purpose of driving athermo-chemical, thermo-mechanical or other thermal process.

BACKGROUND

There exists today considerable interest in harnessing renewable solarthermal energy for a multitude of heat driven processes. These mayinclude thermo-mechanical as in sterling engine or steam turbine powergeneration systems, thermo-chemical reforming, thermal-cracking, processheating, general heating, materials processing etc. Solar collectionsystems are usually placed in locations where sunlight is readilyavailable. In a typical system mirrors either flat-segmented, or curved,are arranged in a parabolic or trough configuration to concentrateincident solar radiation on a predefined target. Tracking controlsystems or preprogrammed algorithms maintain the required opticalgeometry by moving the mirror as the sun transverses the sky.

The target is usually some form of cavity or shallow dish into which theconcentrated light cone is directed. The cavity is commonly disposedwith a plurality of tubes into which a coolant is flowed to conveyabsorbed heat to the working process. Some cavity designs as in U.S.Pat. No. 5,113,659 incorporate a series of hot shoes inside a cavity toconduct thermal energy to a plurality of free piston sterlinggenerators. In some solar thermo-chemical processing the image fireballis employed to directly heat catalyst beds in transparent process tubesoften resulting in hotspots, causing catalyst sintering and poor processtemperature control.

In all these collection schemes, the spot size and shape must betailored for the heat exchange and cavity parameters. To avoid localoverheating effects the fireball is often defocused or multiple fireballimages are skewed to provide a homogenous heat zone into which theprocess heat exchange tubes are displaced. This results in a less thanoptimal focus of the solar fireball on the target and an increase inradiation losses due to the enlarged solar image size with theaccompanying increased area of hot radiating surfaces.

Scaling and the costs of solar collection technology will be dictated toa large part by overall product conversion efficiency, therefore thegoal of any solar collection system is the maximum product productionfor the smallest possible solar collection area. A key factor inachieving this goal is the minimization of parasitic losses due totarget re-radiation.

The required process temperatures dictate the collection means, be ittrough reflectors for low-grade heat applications or parabolicconcentrators for higher temperatures. Steam systems may be operated atmoderate temperatures of less than 800 K, whereas thermo-chemistry in aneffort to obtain high equilibrium constants in some endothermicreactions may require substantially higher temperatures. Unfortunatelyas process temperatures increase, parasitic radiation loss followsStefan's Law (Pr=σεAT⁴) such that losses due to thermal radiationincrease sixteen fold for each doubling of the absolute temperature ofthe target, which is at the process temperature. It follows that minimumradiation loss can be realized by utilizing the smallest possiblefireball image or the highest solar concentration in conjunction with anoptimized cavity receiver configuration in which the blackbody areaequals the focused solar image.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a solar heatcollecting apparatus and method that overcomes problems in the priorart.

In a first embodiment the invention provides an apparatus for collectingheat from a solar concentrator and for transferring the collected heatto a heat sink. The apparatus comprises an isothermal body defining anelongated cavity with a substantially circular opening having a diametersubstantially equal to a diameter of a focus of the solar collector, thecavity having reflective walls such that solar rays contacting the wallsare substantially reflected. The isothermal body is adapted to beoriented such that the circular opening is located substantially at thefocus of the solar collector and substantially perpendicular to aprincipal axis of the solar concentrator, and such that an axis of thecavity is substantially aligned with the principal axis of the solarconcentrator. The isothermal body is adapted for thermal connection tothe heat sink such that heat generated in the isothermal body isabsorbed by the heat sink. The length of the cavity is sufficient toabsorb a desired proportion of the energy in the solar rays entering thecavity.

In a second embodiment the invention provides an apparatus forcollecting heat from the sun and for transferring the collected heat toa heat sink. The apparatus comprises a solar concentrator, and anisothermal body defining an elongated substantially cylindrical cavitywith a substantially circular opening having a diameter substantiallyequal to a diameter of a focus of the solar collector, the cavity havingreflective walls such that solar rays contacting the walls aresubstantially reflected. The isothermal body is oriented such that thecircular opening is located substantially at the focus of the solarcollector and substantially perpendicular to a principal axis of thesolar concentrator, and such that an axis of the cavity is substantiallyaligned with the principal axis of the solar concentrator. Theisothermal body is adapted for thermal connection to the heat sink suchthat heat generated in the isothermal body is absorbed by the heat sinkand a length of the cavity is about 5 to 9 times the diameter of thecircular opening.

In a third embodiment the invention provides a method for collectingheat from a solar concentrator for transfer to a heat sink. The methodcomprises providing an isothermal body defining an elongated cavity witha substantially circular opening having a diameter substantially equalto a diameter of a focus of the solar collector, the cavity havingreflective walls such that solar rays contacting the walls aresubstantially reflected; orienting the isothermal body such that thecircular opening is located substantially at a focus of the solarcollector and substantially perpendicular to a principal axis of thesolar concentrator, and such that an axis of the cavity is substantiallyaligned with the principal axis of the solar concentrator; reflectingeach solar ray that contacts a reflective wall from a first contactpoint on the reflective wall to a second point on a reflective wall andto a plurality of subsequent contact points on the reflective wallswherein a portion of the energy contained in each solar ray is absorbedby a reflective wall at each contact point until a desired proportion ofthe energy contained in the solar ray is absorbed by the reflectivewalls; thermally connecting the heat sink to the isothermal body suchthat heat generated in the isothermal body by the absorbed energy of thesolar rays is absorbed by the heat sink.

The solar radiation is converted to heat by multiple internalreflections within the reflective cavity disposed in the isothermalbody, and this cavity receiver assembly is thermally coupled to therequired heat process or heat sink. the isothermal body has significantmass to integrate thermal fluctuations and provide the coupled processwith a substantially consistent temperature regardless of minorinsulation or fireball image deviations.

The cavity opening is positioned at the focus of a parabolic solarconcentrator on the principal optical axis such that the light cone isat its minimum diameter at the cavity entrance.

The mechanical configuration resembles a thick walled hollow cylinderclad with or constructed wholly of a chemically reducible material suchas, but not limited to, copper. The isothermal body is thermally coupledto a heat process while the open end of the cavity intercepts the lightcone at the foci from a solar concentrator. Solar flux enters the cavityand undergoes multiple internal reflections while evenly dispersing andgradually reducing the radiation to heat which is absorbed by theisothermal body of the receiver and conducted to the process.Reflectivity of the cavity walls is maintained by an inert or reducinglocal atmosphere.

DESCRIPTION OF THE DRAWINGS

The aforementioned objects and advantages of the present invention aswell as additional objects and advantages thereof will be more fullyunderstood herein as a result of a detailed description of preferredembodiments of the invention when taken in conjunction with thefollowing drawings where like components in the drawings are assignedlike designators and where:

FIGS. 1 and 2 are schematic views of a prior art solar heat collectionsystem configured to drive a Sterling to electrical converter,

FIG. 3 depicts the target irradiance profile of a radially skewed solarcollector used in the prior art to reduce solar concentration to anacceptable level;

FIG. 4 is a graph of the radial flux distribution of the prior art andthe current invention;

FIG. 5 is a graphical representation of blackbody thermal radiation lossin relation to target temperature and area;

FIG. 6 is a schematic sectional side view of an embodiment of thepresent invention employed in a Sterling engine driven generator system;

FIG. 7 is a schematic sectional side view of an alternate embodiment ofthe invention in a superheating application;

FIGS. 8 and 9 are schematic sectional side views of an embodiment of thepresent invention employed in a thermo-chemical reactor system;

FIG. 10 is a schematic isometric view of an isothermal body of theinvention defining a co-axial cavity and illustrating a single ray pathand the basic principals of the cavity operation;

FIG. 11 is an end view of the thermally conductive body of FIG. 10illustrating the internal ray path.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIGS. 1 and 2 schematically illustrate a prior art system of solar heatcollection driving a heat engine and electric generator combination.Solar heat collection systems are used to provide heat for a widevariety of purposes in which the collected heat is transferred to a heatsink, such as the illustrated heat engine, that essentially consumes theheat. The operating temperature will vary depending on the purpose, andthe system will be designed such that all the collected heat will bedrawn away by the heat sink once the operating temperature is at thedesired temperature which can vary from about 100° C. to 1400° C. ormore.

In this example solar radiation 1 is reflected by the solar concentrator2 in solar rays 7 of a solar beam and focused on a target 8 positionedin a cavity 11 at the focus of a parabolic concentrator 2. The target 8consists of a plurality of metal tubes 3 arranged symmetrically aboutthe principal axis 9 of the parabolic solar concentrator 2 to interceptthe light cone. To reduce thermal convection losses a quartz window 5covers the target 8. A coolant flows through the tubes 3 to remove heatgenerated by the absorption of radiation on the tubes 3 and transferthis heat to the heat engine 4 by conduction.

The mechanical energy converted by the heat engine in this example iscommunicated by a shaft 10 to a generator 6, which converts themechanical energy to electrical energy. In this example, the fluxdistribution directed at target 8 conforms to an annular ring as shownin FIG. 3 by skewing multiple fireball images on the tubular heatexchange structure in an effort to reduce solar flux intensity levels tothe heat exchange design limits. In FIG. 4, curves W graphicallyillustrate the resulting radial flux distribution at the target 8 causedby the superimposed and skewed fireball images of FIG. 3. As seen inFIG. 3, an approximately circular portion in the middle of the target issubstantially not exposed to the solar rays 7. The concentration of thesolar beam 7 on the target 8 is thus reduced by increasing the radiatedtarget area.

Solar concentration is typically measured in units of “suns”. One sunrepresents the energy incident upon a unit area normal to the sun, whichis about 1000 watts per square meter (W/m²). Further for example, whilethe solar concentration possible at the focus might be about 5500 suns,the heat exchange tubing 3 will not withstand the heat developed at thatconcentration. Given the heat capacity and mass flow of the coolant,along with thermal transfer parameters of the heat exchange, the maximumsafe solar concentration in this example is limited to about 877 suns or877000 W/m². To reduce the solar concentration the mechanism isarranged, by skewing the parabolic concentrator 2 for example, so that alarger area is radiated, and the solar concentration is thus reduced, toeffect the required thermal transfer while maintaining the temperatureof the exchanger within design limits.

Increasing the target size however also increases the radiation lossesat a given temperature and reduces the efficiency of the solarcollector. As shown in FIG. 5, the magnitude of energy loss at anemissivity of 1.0 due to target re-radiation is substantially affectedby the process temperature and the radiant area of the target. In theexample above in FIGS. 1 and 2, the diameter of the target 8 would beabout 15 inches and the solar concentration is 877 suns on a target areaof about 177 square inches (including the circular portion in the middleof the target that is substantially not exposed to the solar rays 7). Byrearranging the parabolic solar concentrator 2 to concentrate a singlefireball at the focus, the target can have a diameter of about 6 inchesso that the solar concentration is 5500 suns on a target area of about28 square inches at the focus. Thus the target area is reduced by afactor of about 6.25 and the concentration correspondingly increases bya factor of 6.25.

As seen in FIG. 5, by decreasing the target size to six inches from 15inches, the radiation loss can be reduced from 13% to 2% where theprocess operating temperature is 850° C. As seen in FIG. 5, theradiation losses for the larger target increase dramatically as theoperating temperature rises, while the radiation losses for the smallertarget increase much less. These data are thermodynamic realitiesconsistent with any blackbody solar receiver design at the indicatedtemperatures. The much higher solar concentration however is problematicwhen actually building a collector of such a small diameter.

FIG. 6 illustrates an embodiment of an apparatus of the invention forcollecting heat from a solar concentrator and for transferring thecollected heat to a heat sink. The heat sink in the illustratedembodiment is a Sterling engine-generator similar in design to that ofFIGS. 1 and 2 with a collection apparatus of the present invention.Here, instead of skewing the concentrator, solar radiation 1concentrated by a parabolic solar concentrator 2 is sharply focused to asingle fireball image at the entrance opening of elongated cavity 13.Here the solar concentration is greatest, and the diameter of thetarget, the entrance opening of the cavity 13, is smallest. In FIG. 4,curve S graphically illustrates the resulting radial flux distributionat the target 8 with a single fireball image.

The opening of the cavity 13 is circular having a diameter substantiallyequal to the diameter of the focus of the solar collector 2. The cavity13 is oriented such that the circular opening is located at the focus ofthe solar collector 2 and substantially perpendicular to a principalaxis 9 of the solar concentrator 2, and such that an axis of the cavity13 is substantially aligned with the principal axis 9.

The cavity 13 is defined in an isothermal body 12 made from stainlesssteel, or the like. The cavity 13 is lined with a metal liner 32 such ascopper exhibiting good reflectivity in a chemically reduced state andexcellent thermal conductivity. Alternatively, the isothermal body 12may be constructed wholly of a chemically reducible and thermallyconductive material such as but not limited to copper. In any event thecavity 13 has reflective walls such that solar rays 7 contacting thewalls are substantially reflected. Multiple reflections of the lightbeam within the cavity 13 transform the energy from the solar rays toheat in the walls of the cavity 13 that is transferred by conduction tothe isothermal body 12 increasing its temperature and making this heatenergy available to the heat sink process.

By reflecting solar rays 7 that contact a reflective wall from a firstcontact point on the reflective wall to a second point on a reflectivewall and to a large plurality of subsequent contact points on thereflective walls the effective area of the receiver is increased fromthe area of the opening of the cavity to the area of the walls of thecavity. Since the cavity is elongated compared to the opening of thecavity, the proportion of solar rays that reflect from wall to wall andthen out through the opening before being absorbed is small.

The proportion of solar energy absorbed can be increased by increasingthe length of the cavity. Total absorption of the beam is unrealistic,however if the length of the reflective cavity 13 is about 5-9 times thediameter of the cavity entrance opening the length of the cavity willgenerally be sufficient to absorb a desired significant proportion ofthe solar rays entering the cavity. Tests have shown a very goodapproximation of a blackbody absorber is realized with minimal blackbodyarea where the length of the reflective cavity 13 is about 7 times thediameter of the cavity entrance opening. With such a configuration about95% of the solar energy is absorbed.

Increasing the length of the cavity 13 will increase the proportion ofsolar rays absorbed, however the length of the isothermal body 12 isalso increased. As the size of the isothermal body 12 increases,conductive heat losses from the isothermal body increase as well andgains in radiation reduction are offset by conduction losses through theenlarged surface area of the isothermal body 12. Decreasing the lengthof the cavity 13 will result in a reduced proportion of the energy inthe solar rays 7 being absorbed, as a greater proportion of the rayswill be reflected out of the cavity 13 and lost.

The cavity 13 is maintained in a reducing local atmosphere for thechemical reduction of exposed metallic components whose reflectivitywould decrease if oxidized and thus reduce the effectiveness of theapparatus.

FIGS. 10 and 11 illustrate the isothermal body 12 and cavity 13 of thepresent invention excluding any heat extraction means where a single raypath of the solar beam 7 is traced through the entrance 20 of the cavity13 and encounters the reflective wall of the cavity 13. The solar ray 7or photon in this example undergoes many reflections before finallybeing absorbed by the cavity wall where its energy is transferred to theisothermal body 12 thus increasing its internal energy or temperature.The path followed by the photon in FIGS. 10 and 11 is but one of amyriad of paths possible, depicted for illustrative purposes only.Focused light energy with a Gaussian beam profile directed at the cavityentrance would follow every possible path within the cavity evenlydistributing the heat therein.

As the temperature of the isothermal body 12 increases, the exposed face14, depending on its emissivity and area, will radiate energycontributing to the total parasitic loss. It is advantageous thereforeto construct a shield 30 of a similar reducible material such as copper,as illustrated in FIGS. 6-9, to cover this or any exposed face of theisothermal body 12 between the opening of the cavity and the outer edgesof the isothermal body in an effort to reduce the thermal radiation lossby reducing surface emissivity. Chemically reducible and similar shieldscan be used to cover any exposed components at the process temperature.

FIG. 7 shows a superheating arrangement used for steam or working fluidsof a heat process. Solar energy as described in the aforementioned isabsorbed by the mechanism of multiple internal cavity reflections andabsorption, which heat the isothermal body 12 to the process temperaturerequired. The working fluid enters the receiver at 18 and is circulatedcyclically through passages 17, symmetrically located in the isothermalbody 12, absorbing energy from the body and exiting to the requiredprocess at 19.

In the embodiments of FIGS. 6 to 9, a sealed enclosure 16 is providedwhich serves to contain a reducing atmosphere 15 as well as any requiredinsulation. A window 5 allows entry of solar radiation to the reflectivecavity 13 and also provides a gas seal for the enclosure 16. Theenclosure 16 is filled with a reducing atmosphere, such as 5% hydrogenand the balance a filler gas that is inert at the operating temperature.Nitrogen is a good choice since it is cheap and inert at higheroperating temperatures. Other inert gases such as argon, etc, could beused as well. The reducing atmosphere maintains the reducible metals,for example oxygen free high conductivity (OFHC) copper or other likemetallic compounds, in their required metallic form. In this state, thereflective surfaces of the liner 32 of reflective cavity 13 and shield30 maintain a low emissivity thereby fulfilling their function in thisinvention.

To reduce heat loss the enclosure 16 containing the reducing gas isinsulated.

In FIGS. 6-9, as solar radiation heats the cavity 13, thermal expansionof the metal cavity liner 32 forming the metallic walls of the cavity 13causes high compression forces against the interior walls of theisothermal body 12. Intimate contact between these components decreasesthe thermal resistance of the metallic boundary between the liner 32 andthe isothermal body 12 enhancing thermal transfer to the heat receivingisothermal body 12, increasing the maximum rated flux density of thecavity by making the cavity liner and receiver assembly substantiallyisothermal.

FIGS. 8 and 9 illustrate a thermo-chemical solar reactor whereconcentrated solar beam 7 enters a gas sealed enclosure 16 through aquartz window 5 where, through multiple cavity reflections, the energyof the solar beam 7 is absorbed by the isothermal body 12 and convertedto heat. Reactant gas is admitted to the feed line 22 and preheatchannel 24. The hot reactant, on exiting the preheater channels 24,enters the reaction beds 25 within the isothermal body where a catalyzedendothermic reaction occurs. The products in these examples exit theisothermal body at tubes 23.

Other embodiments of the examples depicted in FIGS. 6 through 9 wouldinclude a solid isothermal body constructed of a reducible metal orceramic thereby eliminating the need for a reflective cavity liner orshield. Other means of inhibiting oxidation of these key components suchas other reducing gasses or varying concentrations of the prescribedgasses are contemplated within the scope of this invention.

The apparatus of the present invention is suitable for use with higheroperating temperatures where radiation losses represent a significantportion of collected solar energy. At lower operating temperatures, theradiation losses are less significant and use of the apparatus will nottypically provide significant benefits.

Thus the foregoing is considered as illustrative only of the principlesof the invention. Further, since numerous changes and modifications willreadily occur to those skilled in the art, it is not desired to limitthe invention to the exact construction and operation shown anddescribed, and accordingly, all such suitable changes or modificationsin structure or operation which may be resorted to are intended to fallwithin the scope of the claimed invention.

1. An apparatus for collecting heat from a solar concentrator and fortransferring the collected heat to a heat sink, the apparatuscomprising: an isothermal body defining an elongated cavity with asubstantially circular opening having a diameter substantially equal toa diameter of a focus of the solar collector, the cavity havingreflective walls such that solar rays contacting the walls aresubstantially reflected; wherein the isothermal body is adapted to beoriented such that the circular opening is located substantially at thefocus of the solar collector and substantially perpendicular to aprincipal axis of the solar concentrator, and such that an axis of thecavity is substantially aligned with the principal axis of the solarconcentrator; wherein the isothermal body is adapted for thermalconnection to the heat sink such that heat generated in the isothermalbody is absorbed by the heat sink; and wherein a length of the cavity issufficient to absorb a desired proportion of the energy in the solarrays entering the cavity.
 2. The apparatus of claim 1 wherein theproportion of the energy in the solar rays entering the cavity that isabsorbed increases as the length of the cavity increases.
 3. Theapparatus of claim 1 wherein the length of the cavity is about 5 to 9times the diameter of the circular opening.
 4. The apparatus of claim 3wherein the length of the cavity is between 6.5 to 7.5 times thediameter of the circular opening.
 5. The apparatus of claim 1 whereinthe cavity is substantially cylindrical.
 6. The apparatus of claim 1wherein the isothermal body is made from a reflective material such thatthe walls of the cavity are reflective.
 7. The apparatus of claim 1comprising a liner made of reflective material between the isothermalbody and the cavity and operative to provide the reflective walls of thecavity.
 8. The apparatus of claim 7 further comprising a low-emissivityshield covering an end of the isothermal body between the opening of thecavity and outer edges of the isothermal body.
 9. The apparatus of claim1 further comprising an enclosure enclosing the isothermal body, and areducing atmosphere inside the enclosure operative to substantiallyprevent oxidation of the reflective walls of the cavity and therebymaintain reflectivity of the reflective walls.
 10. The apparatus ofclaim 9 wherein the reflective walls comprise OFHC copper and whereinthe reducing atmosphere contains hydrogen and a filler gas.
 11. Theapparatus of claim 9 further comprising insulation in walls of theenclosure.
 12. An apparatus for collecting heat from the sun and fortransferring the collected heat to a heat sink, the apparatuscomprising: a solar concentrator; an isothermal body defining anelongated substantially cylindrical cavity with a substantially circularopening having a diameter substantially equal to a diameter of a focusof the solar collector, the cavity having reflective walls such thatsolar rays contacting the walls are substantially reflected; wherein theisothermal body is oriented such that the circular opening is locatedsubstantially at the focus of the solar collector and substantiallyperpendicular to a principal axis of the solar concentrator, and suchthat an axis of the cavity is substantially aligned with the principalaxis of the solar concentrator; wherein the isothermal body is adaptedfor thermal connection to the heat sink such that heat generated in theisothermal body is absorbed by the heat sink; and wherein a length ofthe cavity is about 5 to 9 times the diameter of the circular opening.13. The apparatus of claim 12 further comprising a low-emissivity shieldcovering an end of the isothermal body between the opening of the cavityand outer edges of the isothermal body.
 14. The apparatus of claim 12further comprising an enclosure enclosing the isothermal body, and areducing atmosphere inside the enclosure operative to substantiallyprevent oxidation of the reflective walls of the cavity and therebymaintain reflectivity of the reflective walls.
 15. The apparatus ofclaim 14 wherein the reflective walls comprise OFHC copper and whereinthe reducing atmosphere contains hydrogen and a filler gas.
 16. A methodfor collecting heat from a solar concentrator for transfer to a heatsink, the method comprising: providing an isothermal body defining anelongated cavity with a substantially circular opening having a diametersubstantially equal to a diameter of a focus of the solar collector, thecavity having reflective walls such that solar rays contacting the wallsare substantially reflected; orienting the isothermal body such that thecircular opening is located substantially at a focus of the solarcollector and substantially perpendicular to a principal axis of thesolar concentrator, and such that an axis of the cavity is substantiallyaligned with the principal axis of the solar concentrator; reflectingsolar rays that contact a reflective wall from a first contact point onthe reflective wall to a second point on a reflective wall and to aplurality of subsequent contact points on the reflective walls until adesired proportion of the energy contained in the solar rays is absorbedby the reflective walls; thermally connecting the heat sink to theisothermal body such that heat generated in the isothermal body by theabsorbed energy of the solar rays is absorbed by the heat sink.
 17. Themethod of claim 16 wherein the proportion of the energy in the solarrays entering the cavity that is absorbed increases as the length of thecavity increases.
 18. The method of claim 16 wherein the cavity issubstantially cylindrical and the length of the cavity is about 5 to 9times the diameter of the opening of the cavity.
 19. The method of claim16 comprising enclosing the isothermal body in an enclosure andproviding a reducing atmosphere inside the enclosure operative tosubstantially prevent oxidation of the reflective walls of the cavityand thereby maintain reflectivity of the reflective walls.
 20. Themethod of claim 19 wherein the reflective walls comprise OFHC copper andwherein the reducing atmosphere contains hydrogen and a filler gas.